1. Field of the Invention
The present invention relates to a semi-cured product, a cured product and a method for producing these, and to an optical component and a curable resin composition.
2. Description of the Related Art
Heretofore, in general, inorganic glass materials have been used for optical components (also referred to as optical elements, mainly lenses) from the viewpoint that they are excellent in optical characteristics, mechanical strength, etc.
Recently desired is further improvement of production efficiency in a production system for image-taking devices through solder reflow treatment of optical modules with optical elements integrated therein in a state where those optical elements are mounted in addition to other electronic components on a circuit board.
Naturally, also in an optical module to be produced in the production system with reflow treatment incorporated therein, it is desired to use plastic optical elements capable of being produced at low cost rather than high-cost glass optical elements.
However, thermoplastic resins that have heretofore been used as resin materials for optical elements soften and melt at relatively low temperatures and therefore have good workability, but the optical elements molded from them are defective in that they readily deform by heat. In a case where electronic components with optical elements incorporated therein are mounted on a substrate in a solder reflow process, the optical elements themselves are also exposed to heating conditions, for example, at 270° C. or so; but in such a case, optical elements formed of a thermoplastic resin having poor heat resistance would be problematic as causing deformation or shape degradation. Given the situation, it is desired to impart heat resistance enough for reflow treatment to plastic optical components and to arrange them on a substrate simultaneously with other electronic elements thereon to thereby reduce the production cost.
Regarding the above, there is known a method of using a photocurable resin as the plastic material for optical elements for use in image-taking devices to be produced according to a reflow process (for example, see PATENT DOCUMENT 1). In general, a photocurable resin is, before cured, liquid or flowable and has good workability like a thermoplastic resin, but after cured, it does not exhibit flowability like that of a thermoplastic resin and therefore deforms little by heat. However, since the viscosity thereof before cured is low, a photocurable resin has a problem in that it leaks out through the parting line of a mold during molding therein and the leaked resin also cures to form an unnecessary part during molding. The leaked resin forms a thin film of a so-called burr and forms other rod-shaped, spherical or horn-like projections, but in this description, these are all collectively referred to as “burrs”.
For suppressing mold clearance leakage so as to prevent burr formation during molding, PATENT DOCUMENT 1 proposes a production method that comprises semi-curing a curable composition through UV irradiation to form a semi-cured product (gummy product) and pressing and thermally curing it in a mold. Concretely, the patent reference discloses in the section of Examples therein, a case where a curable resin composition comprising a 2-alkyl-2-adamantyl(meth)acrylate as a curable resin and an ordinary thickener, sodium polyacrylate (or sulfonic acid copolymer), and a photopolymerization initiator and a thermal polymerization initiator added thereto is thickened through photoirradiation and then cured under heat to reduce burr formation during molding.
On the other hand, there is known a method of adding various additives to a photocurable resin composition to change the properties of the composition after cured. For example, PATENT DOCUMENT 2 discloses an embodiment of using a curable resin composition prepared by adding to an acrylic resin, from 0.5 to 30 parts by mass of a monofunctional acryl, a difunctional acryl or a terpene compound, and a photopolymerization initiator, as a coating resin composition and a COP-protective coating material for molded articles of an alicyclic structure-containing polymer (COP). The patent reference discloses use of a resin prepared by polymerizing a terpene compound for enhancing adhesiveness to COP. However, in the patent reference, there is given no description to suggest use of a thermal polymerization initiator, and the above-mentioned composition could not be used for heat-resistant lenses directly usable in a reflow process.
PATENT DOCUMENT 3 discloses in Example 6 therein a case of adding an alicyclic radical-polymerizing group-containing compound (A), a photopolymerization initiator and a thermal polymerization initiator (B) and a difunctional radical-polymerizing compound (A′) and photopolymerizing and thermally-polymerizing them in that order. The patent reference discloses the ability to solve the problem of microfabrication with the curable resin composition having the constitution as above. However, from the viewpoint of application to optical components such as heat-resistant lenses, the disclosed composition is still unsatisfactory in point of the fabricability thereof.